one of these, an infant, flew on a plane to hawaii where she was intercepted and quarantined. her fellow 250 passengers had to be contacted and tracked. dr. wilma wooten is san diego county's public health officer. >> this entire process resulted in exposures of almost 1,000 individuals-- 90 individuals having no proof of immunization, 73 were quarantined, 12 additional actual cases of measles in san diego. >> narrator: public health officials determined that what allowed measles to enter the community was the number of vaccine-hesitant parents who took advantage of the personal belief exemption allowed by california. school principals find themselves caught on the frontlines. >> the fact is that some families choose not to immunize their children. and then there are families who have children who are particularly medically sensitive, and they're in jeopardy because they could get sick from unimmunized children. so it's a very emotional issue on both sides. >> narrator: a vast public health infrastructure is committed to preventing such outbreaks. the national institutes of he